Deep Link: A deep link is any hyperlink that points to an internal page on a website. When used on a webmaster's own website, these links help direct users to other resources on a particular site; they also help the users stay on the site in order to increase click throughs. Deep links can also be used by another site to point to a specific source page on another website. Deep links do not apply to links to a homepage (e.g. www.example.com) but to a specific page (e.g. www.example.com/page.html).
Most higher quality websites will have links to their internal pages. Wikipedia, for instance, has millions of internal deep links to other parts of the site. However, smaller sites can also include deep links in blog posts and on informational pages in order to create more inbound links to a page.
Some sites avoid deep linking because they prefer their links to direct to a main page. This issue is further complicated by the fact that some sites will charge for access to permanent links, which is especially the case on news sites. Other websites may delete older content, thus rendering an older link useless.
Because deep links are often indexed by search engines, some websites will opt out of this service in its robots.txt file. However, this won't necessarily prevent other sites from linking to a page's content.